The present invention relates to an electronic engine control system and a method which utilizes an exhaust emissions sensor and adaptively maintains a given emissions level.
Engines exhibit a great deal of variability in terms of emissions output on an engine-to-engine basis. This variability is a natural consequence of the manufacturing variability present in the engine components and sensors used in the control system. This variability creates a problem when engine developers attempt to reach increasingly lower emissions targets. Since the emissions variation must be taken into account, the mean emissions level is forced to be lower than necessary so that the engine that produces emissions higher than the mean remains below the required emissions standard.
For example, if the emissions certification standard is 2.5 g/bhp-hr for NOx emissions, and the amount of variability from engine to engine is 1.0 g/bhp-hr, then the development target for emissions must be 1.5 g/bhp-hr. This reduction in the development target reduces the overall efficiency of the engine. A significant reduction of the variability between engines would allow engines to achieve the same emissions level but with higher efficiency.
U.S. Pat. No. 5,809,967, issued in 1998 to Masubuchi, describes a system which controls ignition timing as a function of sensed exhaust oxygen concentration and sensed exhaust air-fuel ratio. This system is intended to compensate for variations in the fuel supplied to the engine, not to reduce variability of exhaust emissions.
Accordingly, an object of this invention is to provide a system and a method for reducing the level of emissions variability on engines.
This and other objects are achieved by the present invention, wherein a system and method of controlling an engine includes sensing a concentration of an emission species in exhaust gases produced by the engine and generating a sensed emission species value representative thereof, and controlling engine control system set points, such as spark timing, air/fuel ratio, boost, intake temperature or load, as a function of the sensed emission species value. The method also includes calculating a emission species difference value by subtracting a base emission species value from the sensed emission species value, and if an absolute value of the difference value exceeds a threshold, modifying the engine control system set point as a function of the emission species difference value. The method also includes sensing engine RPM, sensing engine manifold air pressure (MAP), determining the base emission species value as a function of sensed engine RPM and MAP. The method may also include sensing humidity in engine intake air, and correcting the base emission species value as a function of the sensed humidity. The method may also include calculating engine air flow, and correcting the base emission species value as a function of the calculated air flow.